The present invention relates to manufacturing optical articles out of thermoplastic synthetic material, such as ophthalmic lenses, instrument lenses or precision optics, as obtained by injection molding.
The molding of ophthalmic lenses out of thermoplastic synthetic material is usually performed by injection molding, with this method enabling raw plastics material to be transformed directly into finished lenses (excluding coatings). In the manufacture of lenses by a method of this kind, it is conventional for the thermoplastic material to be initially heated so as to be molten at a temperature above the vitreous transition point. While in this form, the material is introduced under high pressure into a mold cavity of appropriate dimensions and shape that is formed in a mold. The material is then allowed to cool down so as to solidify, after which the resulting lens is extracted from the mold. Usually, the material used is a thermoplastic resin such as polymethyl methacrylate, polycarbonate, or a copolymer of polycarbonate, polynorbornene, polystyrene, cyclic polyolefins and their copolymers, etc.
To obtain ophthalmic lenses possessing optical qualities suitable for their purpose, certain precautions need to be taken during manufacture, in particular to avoid irregular deformations or the presence of residual internal tensions. Such deformations or tensions give rise to anisotropy or to other undesirable optical aberrations such as double refraction.
In this respect, special care is taken when making the wall of the mold cavity in the mold. Usually, the mold comprises two half-blocks each of which has at least one mold recess formed therein for association with a corresponding recess formed in register in the other block. The two half-blocks can move relative to each other between an open configuration giving direct access to the recesses and a closed configuration in which the two half-blocks come into contact with each other via junction faces lying in a transverse join plane and in which the recesses co-operate in pairs to form the desired mold cavity(ies).
Each mold recess is defined transversely by an interchangeable mold shell which presents a molding working face possessing appropriate dimensions and curvature corresponding to those that are to be imparted to the finished lens (with allowance being made for a certain amount of shrinkage). By way of example, such shells are made of stainless steel, a nickel-based alloy, or mineral glass, and they present optical polish, i.e. polish analogous to that of a mirror.
In addition, it is often recommended to proceed with injection of the material into the recess in two successive stages: a first stage of filling proper during which the recess is filled progressively, and a second stage of overpacking or compression which takes place after the recess has been filled completely. This second stage of overpacking or compression consists in subjecting the material introduced in this way into the recess to high pressure for a given length of time in order to eliminate shrink marks, to ensure that the material has the proper density, and to reduce harmful internal tensions, at least to some extent. When this holding pressure is generated by the injection machine itself, the material is said to be being overpacked. When it is the result of moving the mold shells towards each other, then the material is said to be being compressed.
In any event, these precautions relating to tooling and mode of operation need to be associated with precautions relating to how the plastics material and the mold are heated during molding. It turns out to be essential to have accurate control over the temperature of the mold cavity and its wall, in particular the temperature of the working face of the shell, for this to apply throughout all of the molding steps, and for this to be done using a temperature gradient that is defined both in time and in space. For this purpose, the half-blocks of the mold are usually provided with heat transfer means, and preferably with means that are both-way means (i.e. suitable for exerting both heating functions and cooling functions), in order to regulate the temperature of the mold around the mold cavity from one cycle to the next, and in order to accelerate the removal of heat from the molded lens. By way of example, these means can be channels for circulating a heat-conveying fluid. The heat delivered or extracted by circulating the hot fluid or by electrical resistances spreads through the mass of the half-blocks and, by thermal conductivity, through the mass of the shells, and is communicated via the working faces thereof into the mold cavity and thus into the plastics material while it is being shaped.
This method of heating/cooling nevertheless presents drawbacks in practice, in particular because of the considerable thermal inertia of the half-blocks. Lack of precision is observed both concerning temperature distribution within the mold, around the mold cavity, and concerning how temperature varies over time. Initially, this technique does not ensure that the plastics material is heated in regular and uniform manner as would be desirable to avoid internal tension. Different portions of the mold, and more particularly of the wall of the mold cavity, reach the temperature required for each of the various molding stages only progressively and unevenly. This lack of uniformity in heat transmission also arises during cooling after injection. Furthermore, and above all, the way in which the temperature of the mold cavity varies during the different stages of molding is not controlled in a manner that is sufficiently precise, and this runs the risk of giving rise to the above-mentioned major optical defects or else requires cycle times to be lengthened to an unacceptable extent. Because of this unequal heating and cooling at various points of the cast material, and because of the lack of precision in the temperature gradient over time, the resulting lens can conserve deformations and tensions which can make it unsuitable for the intended optical uses.
To improve the precision with which temperature gradient is controlled, molds have been designed in which the shells themselves are provided with intrinsic both-way means for transferring heat, such as channels for circulating a heat-conveying fluid. Integrating both-way heat transfer means in the main portions of the mold as constituted by the shells makes it possible both to heat and to cool the major portion of the wall of the mold cavity in application of an optimized temperature regulation relationship without it being necessary to move the mold in order to subject it to the action of external heating and/or cooling means. Nevertheless, with usual dispositions of that type, the shells provided for receiving the heating and cooling means are fixed to the half-blocks concerned and it is as a function of this arrangement that the couplings to heating fluid or electricity are organized. In installations of that kind, the operations of installing and removing shells are lengthy and complicated.
In order to avoid such complications while retaining the advantages that result from the heat transfer means acting directly on the shells, proposals were made to implement a shell in two portions:
a base receiving all of the heat transfer means for said shell; and
a removable insert fitted to the base and carrying the molding face, said insert having no intrinsic heat transfer means and being temperature-regulated solely by heat transfer with said base.
The insert is thus interchangeable, and as a result, it can be selected from a preestablished set, or can even be made on demand, as a function of how the desired surface for the lens is defined. The insert is thus easy and quick to install and/or remove since it has no hydraulic or electrical couplings, either with the temperature-regulation source or with the base. Couplings to the temperature-regulation source take place via the base and therefore do not need to be disassembled.
Although that configuration is advantageous in that it combines, at least in theory, the advantage of direct temperature action on the shell with the convenience of having an insert that is interchangeable, it nevertheless turns out to be capable of being improved in terms of the efficiency and the precision of the temperature regulation it makes possible.
The invention provides a mold half-block for injection molding an optical article such as an ophthalmic lens out of thermoplastic material, the half-block possessing at least one mold recess defined transversely by the working face of a shell provided with heat-transfer means that are intrinsic and both-way (both heating and cooling), said shell including a base receiving all of the heat-transfer means of said shell and a removable insert fitted on the base and carrying the working face, said insert having no intrinsic heat-transfer means and being thermally regulated solely by heat transfer with said base. The insert is cap-shaped and fits over a head portion of at least the base, and the heat-transfer means of the base are allocated to the head portion of the base that is covered by the insert.
Such an arrangement makes it possible to improve the efficiency and the precision with which heat is exchanged between the heat-transfer means and the insert via the base. It will be understood that firstly the cap shape of the insert and the engagement of the head portion of the base in said insert of itself increases the heat exchange area between the insert and the base. In addition, and above all, in this context the location of the heat-transfer means within the head of the base amounts, from the point of view of heat-transfer efficiency, to placing said heat-transfer means so to speak xe2x80x9cinsidexe2x80x9d the insert, with the head portion of the base, the only portion concerned by heat transfer, being xe2x80x9cenvelopedxe2x80x9d inside the cap-forming insert.
According to an advantageous characteristic of the invention, the shell is received in a jacket having a cylindrical inside face defining the sides of the recess in addition to the working face of the shell, and is likewise provided with heat-transfer means that are intrinsic and both-way. Enveloping the shells in a jacket that is thermally self-modulated makes it possible to improve the quality and in particular the precision with which the temperature gradient of the mold is regulated around the mold cavity, and as a result in the thermoplastic material also. The jacket thus provides a thermal blanket effect which is additional to its optional function of providing the two shells with mechanical guidance relative to each other when it is desired to perform compression at the end of injection.
It is then advantageous for the intrinsic and both-way heat transfer means of the jacket to be independent from those of the shell.
In a preferred embodiment, the intrinsic and both-way heat-transfer means of the shell and of the jacket comprise respective internal circuits for circulating a heat-conveying fluid.